In so called "demand defrost" heat pump systems an attempt is made to defrost the outdoor heat exchanger flow tubes only when ice or frost has actually formed. Numerous different approaches to sensing frost or ice accumulations have been proposed. One general approach is to sense the temperature difference between the atmospheric air and the outdoor flow tubes when the atmospheric air temperature approaches freezing. The insulating effect of frost on the flow tube reduces heat flow to the refrigerant from the ambient air. Consequently when ice or frost forms the outdoor heat exchanger flow tube surface temperature drops relative to the ambient air temperature. The existence of a predetermined temperature differential between the flow tube surface and ambient air (at a given temperature) signifies that frost or ice is present and the tube should be defrosted.
As the atmospheric air becomes increasingly colder, the temperature differential indicative of ice or frost on the flow tube becomes progressively smaller. Thus, sensing the tube surface temperature with a high degree of accuracy is essential to effectively operating demand defrost heat pump systems at low outdoor temperatures. Achieving such accuracy is difficult because devices used to sense tube surface temperatures are typically exposed to the ambient air, often with the air flowing over the sensor at considerable velocity. The ambient air transfers heat to the sensor device. This heats the sensor and reduces its ability to accurately signal the flow tube temperature. Defrost cycles are thus forestalled when ice or frost has accumulated and is adversely effecting the system.
Temperature sensor devices have been attached to the outdoor tubes in different ways. One technique was to clamp an electrically insulated temperature sensor device directly onto the tube using a hose clamp or spring clip. This assured good thermal contact between the heat exchanger flow tube and the temperature sensor, but placed the sensor in convective heat transfer relationship with the atmospheric air. Inaccurate flow tube temperature readings resulted. To minimize heat gains from the air, heavy insulating tape was sometimes wrapped around the sensor and flow tube. The tape tended to assume the ambient air temperature and adversely influence the sensor output signal.
In other installations the sensors were encapsulated in molded rubber-like plastic bodies. In still other installations sensors were placed in metal housings clamped to the flow tubes. The housing materials were efficient heat conductors and the sensors were stationed in the housings in a body of plastic material. These devices did not produce acceptably accurate flow tube temperature readings.
Temperature sensors and techniques for mounting them in thermal contact with an object are documented in a number of prior art patents. U.S. Pat. Nos. 2,484,585 to Quinn, 3,123,790 to Tyler and 3,357,249 to Bernous, et al. disclose temperature sensors and mounting techniques for the disclosed temperature sensors.
In order to function as required for a demand defrost application the sensors had to be constructed so that they quickly and accurately monitored the flow tube temperature. This required great thermal conductivity. At the same time the sensors had to be unresponsive to ambient air temperature, an attribute of a good insulator. These conflicting requirements were not satisfied in prior art sensor constructions.
The present invention provides a new and improved temperature sensing assembly for sensing the temperature of a refrigerant flow tube in a heat exchanger and which is so constructed and arranged that the temperature measurements are substantially unaffected by ambient air temperatures and flows.